util: T2226: better handle stderr
util: T2226: better handle stderr

Do not agreggate stderr with stdout. So if a command reports
a message on stderr but does not report an error, it will
not be send to the user to confuse him.

Explicitely set encoding to utf-8, which does not change
the code behaviour but simplify the code.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x